Hot melt adhesives are generally based on thermoplastic resins. In accordance with the kinds of thermoplastic resins, hot melt adhesives are largely classified into polyolefin-based, elastomer-based, polyester-based and polyamide-based adhesives. These hot melt adhesives are solid materials that are not evaporated at normal temperature, and are thermally melted to exhibit fluidity when used, thereby allowing various adherends to be bonded instantly. Hot melt adhesives have been utilized in various fields due to many advantages such as pollution-free (no VOC emissions), low risk of fire, high productivity and a wide choice of adherends.
Ethylene/vinyl acetate copolymer (hereinafter, sometimes abbreviated to EVA) is a typical polymer used in polyolefin-based hot melt adhesives. Hot melt adhesives containing such a copolymer as well as a tackifier and a wax have found a wide use in products such as packages because they are inexpensive and are applicable to a wide range of applications by controlling the vinyl acetate content and the molecular weight of EVA which is the main component. However, EVA-based hot melt adhesives have some performance problems, in detail, lack of heat-resistant adhesiveness. This fact impedes broadening of the use of these adhesives to such industrial fields as automobiles and building materials. In detail, these adhesives are unsatisfactory in terms of creep characteristic during prolonged use in a high temperature environment (hereinafter, this characteristic is sometimes abbreviated as “thermal creep resistance”) as well as adhesive strength at high temperatures (hereinafter, this characteristic is sometimes abbreviated as “high-temperature bonding properties”).
Various approaches have been disclosed in order to improve thermal creep resistance and high-temperature bonding properties. For example, the content of a component such as vinyl acetate in EVA is optimized, or an aromatic hydrocarbon resin having a specific composition and a specific molecular weight is added to EVA (Patent Literatures 1 and 2). Alternatively, a maleated polyolefin is added to a composition based on EVA (Patent Literature 3). Still alternatively, an amorphous propylene/ethylene copolymer or propylene/butene copolymer is added to an EVA-based composition satisfying specific properties (Patent Literatures 4 and 5). However, these approaches cannot increase the heat resistance of adhesives sufficiently. It has been also found that increasing heat resistance by, for example, the addition of a crystalline component in turn results in a decrease in flexibility with the deterioration of adhesion due to stress concentration. Another approach for increasing heat resistance is to add a high-molecular weight component. In this case, however, it has been found that melt viscosity is so increased that workability, in particular applicability, is deteriorated.